1. Field of the Invention
The present invention is directed generally to an improvement of a magnetic field generating device employing permanent magnets and used for a magnetic resonance imaging device (hereinafter abbreviated to MRI) for a medical-purpose, and more particularly, to a magnetic field generating device for MRI, the arrangement being such that a plurality of magnetic field adjusting magnetic material segments and/or a plurality of magnetic field adjusting permanent magnet segments having a magnetizing direction opposite to that of permanent magnet assemblies are disposed on the same circles or concentric circles on magnetic pole piece surfaces standing vis-a-vis with an air gap to thereby eliminate a non-uniformity of the magnetic fields which is ascribed particularly to a difference in configuration between yokes in a magnetic circuit.
2. Description of the Prior Art
The medical-purpose magnetic resonance imaging device (MRI) is a tomograph capable of obtaining a sectional image of an object by inserting a part of or all of a subject to be examined into an air gap of a magnetic field generating device for generating intensive magnetic fields and depicting even properties of the tissues thereof.
The magnetic field generating device for MRI requires an air gap enough to insert a part or all of the subject. Besides, it is typically required that an intensive and uniform magnetic field exhibiting an accuracy on the order of 0.005-2.0T and 1.times.10.sup.-4 or less be created in an imaging visual field within the air gap.
One known arrangement of the magnetic field generating device for MRI is that magnetic pole pieces (2), (2) opposite to each other are, as illustrated in FIG. 1, fixed to one ends of a pair of permanent magnet assemblies (1), (1) which employ Fe-B-R series magnets; tabular yokes (4) and (5) are fixed to the other ends thereof and linked to each other via a four lengths of columnar yokes (6); and a static magnetic field is generated within an air gap (7) defined by the magnetic pole pieces (2), (2).
Based on the construction given above, with the intention of creating more stable and uniform magnetic field, for instance, there has been proposed a magnetic field generating device (Japanese Utility Model Laid-Open Publication No. 60-166110), wherein annular projections (3) are shaped on the confronting surfaces of the magnetic pole pieces; or alternatively convex projections are formed at the centers of the confronting surfaces of the magnetic pole pieces.
On the basis of the above-described construction, there has also been proposed a magnetic field generating device (Japanese Utility Model Laid-Open Publication No. 62-112106), wherein the annular projections are provided on the confronting surfaces of the magnetic pole pieces, and a single or a plurality of magnetic field adjusting pieces made of a magnetic material are secured to predetermined portions of the annular projections.
The uniformity of the magnetic field of the magnetic field generating device is, as discussed above, required to have an accuracy on the order of 1.times.10.sup.-4 within a predetermined space but is greatly influenced depending on configurations of the yokes or placement thereof in a magnetic circuit and, in particular, greatly influenced by configurations of the magnetic pole pieces.
For example, as in the case of FIG. 1, not only the configurations of the magnetic pole pieces but also the rectangular tabular yokes (4) and (5) and the columnar yokes (6) exert large influences on the uniformity of the magnetic field. The rectangular tabular yokes (4) and (5) intensity the magnetic field acting in a direction Y rather than in a direction X. whereas the columnar yokes (6) weaken the magnetic field acting in a columnar direction.
A high uniformity is obtained by providing the annular projections on the magnetic pole pieces. If an intensity of the magnetic field is locally increased on the magnetic pole piece according to the configurations and placement of the above-mentioned composing members of the magnetic field so as to provide a higher uniformity, iron pieces may be bonded thereto, as is disclosed in Japanese Utility Model Laid-Open Publication No. 62-112106. An extremely high uniformity can not, however, be obtained simply by bonding the iron pieces to the magnetic pole pieces.
Whereas in an attempt to locally decrease the intensity of the magnetic field, a thinkable measure is to reduce the number of magnetic pole pieces concerned disposed on the portions, i.e., to form holes. It is, however, difficult to reprocess the magnetic pole pieces such as boring after the fabrication has been performed. Desired effects are not necessarily be acquired.